Oh ok thank you for your help because my formation was
Gk, 3 CB's, CDM, 3 CM's, CAM, and 2 STK's so affectively a very narrow 3-1-3-1-2 playing through the middle, I noticed the change happened when I changed defending formation from my original 3 backs a LB, CB, and a RB into a 3 CB defence, I have never heard of "illegal formation" before but after having read about it that clarifies it for me thanks heads for that.
Really appreciate it